1. Field of the Invention
The present invention relates to a substrate which is used in a semiconductor element. The present invention also relates to a process for producing a substrate which is used in a semiconductor element. The present invention further relates to a semiconductor element which uses the above substrate.
2. Description of the Related Art
Japanese Journal of Applied Physics Vol. 37 (1998) Part 2, pp. L1020 discloses a short-wavelength semiconductor laser device which emits laser light in the 410 nm band. This semiconductor laser device is produced as follows. First, a GaN layer is formed on a sapphire substrate, a striped pattern of a SiO2 film is formed on the GaN layer, and a GaN thick film is formed by selective lateral growth from nuclei of growth generated in stripe areas of the GaN layer which are not covered by the striped pattern of the SiO2 mask. Then, a GaN substrate is obtained by separating the GaN thick film from the sapphire substrate. Next, an n-type GaN buffer layer, an n-type InGaN crack prevention layer, an n-type AlGaN/GaN modulation doped superlattice cladding layer, an n-type GaN optical waveguide layer, an n-type InGaN/InGaN multiple-quantum-well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, a p-type AlGaN/GaN modulation doped superlattice cladding layer, and a p-type GaN contact layer are formed on the above GaN substrate. However, the highest output power obtained in the fundamental transverse mode by the above semiconductor laser device is about 30 mW, and the semiconductor laser device is reliable in only the output power range up to 30 mW.
Since the conventional ELOG (epitaxial lateral overgrowth) substrates as above are produced by selective lateral growth from nuclei of growth generated in stripe areas of a GaN layer which are not covered by a SiO2 mask, defects are reduced in the region produced by the selective lateral growth of GaN.
Nevertheless, according to the conventional method as above, the density of the nuclei for growth is high, and therefore the spaces between the nuclei are bridged before the grown nuclei become large. Thus, defects are likely to be produced in the bridged regions. Although the GaN thick film is required to have a certain thickness in order to use the GaN thick film as a substrate, the defect density is increased with increase in the thickness. Even when the defect densities in the bridged regions are low, the defect densities increase with the increase in the thickness. Consequently, it is difficult to form a wide low-defect region. That is, the conventional ELOG substrates have only a narrow low-defect region.
In order to realize a reliable semiconductor laser device, an optical waveguide is required to be formed on a low-defect region of a substrate. Therefore, the conventional ELOG substrates are effective for producing semiconductor laser devices having a narrow stripe structure as disclosed in the aforementioned reference. However, it is impossible to form a reliable semiconductor laser device having a broad stripe structure on the conventional ELOG substrates.
In order to realize a reliable semiconductor laser device having high output power, the semiconductor laser device is required to have a broad stripe structure, and in order to realize a reliable semiconductor laser device having a broad stripe structure, the semiconductor laser device is required to be formed on a GaN substrate which includes a wide low-defect region, instead of the conventional ELOG substrates.
Generally, reliability of every semiconductor element constituted by semiconductor layers formed on a substrate, including the semiconductor laser element, depends on the defect density in the substrate. Therefore, a substrate including a wide low-defect region is required for all types of semiconductor elements.
An object of the present invention is to provide a substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
Another object of the present invention is to provide process for producing a substrate which is used in a semiconductor element, and in which the defect density is low in a wide region.
A further object of the present invention is to provide a semiconductor element which uses a substrate in which the defect density is low in a wide region.
(1) According to the first aspect of the present invention, there is provided a process for producing a substrate for use in a semiconductor element, comprising the steps of: (a) forming a first GaN layer having a plurality of pits at the upper surface of the first GaN layer, where each of the plurality of pits has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm; and (b) forming a second GaN layer by growing a GaN crystal over the first GaN layer until the upper surface of the second GaN layer becomes flattened.
According to the first aspect of the present invention, the density of nuclei for growth can be reduced compared with the conventional substrates, and therefore it is possible to obtain a GaN layer in which the defect density is low in a wide region.
Preferably, the following sequence of operations may be performed once or a plurality of times on the second GaN layer of the substrate obtained by the process according to the first aspect of the present invention. The sequence includes the steps of formation of a plurality of pits at the upper surface of an uppermost GaN layer, and formation of an additional GaN layer by growing a crystal over the uppermost GaN layer until the upper surface of the additional GaN layer becomes flattened, where each of the plurality of pits has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm. When the sequence is repeated, the uppermost GaN layer in which the plurality of pits are formed in each repeat sequence is the second GaN layer formed in the step (b) in the process according to the first aspect of the present invention or the additional GaN layer formed in the preceding repeat sequence. By repeating the above sequence, a GaN layer in which the defect density is further reduced can be obtained.
According to the second aspect of the present invention, there is provided a process for producing a substrate for use in a semiconductor element, comprising the steps of: (a) forming a first GaN layer having a plurality of pits at the upper surface of the first GaN layer, where each of the plurality of pits has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm; (b) forming a second GaN layer by growing a GaN crystal over the first GaN layer under a growth condition under which facets each making an angle of 20 to 70 degrees with the upper surface of the first GaN layer are formed, until a ratio of a total area of portions of the upper surface of the second GaN layer which are parallel to the upper surface of the first GaN layer to a total area of the entire upper surface of the second GaN layer becomes 30% or smaller; and (c) forming a third GaN layer by growing a GaN crystal over the second GaN layer until the upper surface of the third GaN layer becomes flattened.
According to the second aspect of the present invention, the density of nuclei for growth can be reduced compared with the conventional substrates, and therefore it is possible to obtain a GaN layer in which the defect density is low in a wide region.
Preferably, the following sequence of operations may be performed once or a plurality of times on the second GaN layer of the substrate obtained by the process according to the first aspect of the present invention. The sequence includes the steps of: formation of a plurality of pits at the upper surface of an uppermost GaN layer; formation of a first additional GaN layer by growing a GaN crystal over the uppermost GaN layer under a growth condition under which facets each making an angle of 20 to 70 degrees with the upper surface of the uppermost GaN layer, until a ratio of a total area of portions of the upper surface of the first additional GaN layer which are parallel to the upper surface of the uppermost GaN layer to a total area of the upper surface of the first additional GaN layer becomes 30% or smaller; and growth of a second additional GaN layer over the first additional GaN layer until the upper surface of the second additional GaN layer becomes flattened, where each of the plurality of pits has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm. When the sequence is repeated, the uppermost GaN layer in which the plurality of pits are formed in each repeat sequence is the third GaN layer formed in the step (c) in the process according to the second aspect of the present invention or the second additional GaN layer formed in the preceding repeat sequence. By repeating the above sequence, a GaN layer in which the defect density is further reduced can be obtained.
As explained above, when a substrate for use in production of a semiconductor element is produced by the processes according to the first or second aspect of the present invention or one of their preferable modifications, the substrate includes a GaN layer in which the defect density is low in a wide region. Therefore, the reliability of semiconductor elements formed on substrates which are produced by the processes according to the first and second aspects of the present invention and their preferable modifications is high.
(2) According to the third aspect of the present invention, there is provided a semiconductor element comprising: a substrate produced by the process according to the first aspect of the present invention; and at least one semiconductor layer formed on the substrate.
According to the fourth aspect of the present invention, there is provided a semiconductor element comprising: a substrate produced by the process according to the second aspect of the present invention; and at least one semiconductor layer formed on the substrate.
Since, in the semiconductor element according to the third or fourth aspect of the present invention, at least one semiconductor layer is formed on a substrate produced by the process according to the first or second aspect of the present invention, the defect density in the substrate is low. Therefore, the reliability of the semiconductor elements according to the third and fourth aspect of the present invention is high.
(3) According to the fifth aspect of the present invention, there is provided a substrate for use in a semiconductor element, comprising: a first GaN layer having a plurality of pits each of which has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm; and a second GaN layer being formed by crystal growth over the first GaN layer and having a flattened upper surface.
Since the substrate according to the fifth aspect of the present invention is produced by the process according to the first aspect of the present invention, the second GaN layer includes a wide region in which the defect density is low.
In a variation of the substrate according to the fifth aspect of the present invention, a plurality of pits each of which has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm are formed at the upper surface of the second GaN layer, and a third GaN layer is formed by crystal growth over the second GaN layer so that the upper surface of the third GaN layer is flattened. In this variation, the uppermost (third) GaN layer includes a wide region in which the defect density is further lower.
According to the sixth aspect of the present invention, there is provided a substrate for use in a semiconductor element, comprising: a first GaN layer having a plurality of pits each of which has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm; a second GaN layer being formed by crystal growth over the first GaN layer and having facets each of which makes an angle of 20 to 70 degrees with the upper surface of the first GaN layer, where a ratio of a total area of portions of the upper surface of the second GaN layer which are parallel to the upper surface of the first GaN layer to a total area of the upper surface of the second GaN layer is 30% or smaller; and a third GaN layer being formed by crystal growth over the second GaN layer and having a flattened upper surface.
Since the substrate according to the sixth aspect of the present invention is produced by the process according to the second aspect of the present invention, the third GaN layer includes a wide region in which the defect density is low.
In a variation of the substrate according to the sixth aspect of the present invention, a plurality of pits each of which has an opening area of 0.005 to 100 xcexcm2 and a depth of 0.1 to 10.0 xcexcm are formed at the upper surface of the third GaN layer, a fourth GaN layer having facets each of which makes an angle of 20 to 70 degrees with the upper surface of the first GaN layer is formed by crystal growth over the third GaN layer, and a fifth GaN layer is formed by crystal growth over the fourth GaN layer so that the upper surface of the fifth GaN layer is flattened, where a ratio of a total area of portions of the upper surface of the fourth GaN layer which are parallel to the upper surface of the third GaN layer to a total area of the upper surface of the fourth GaN layer is 30% or smaller. In this variation, the uppermost (fifth) GaN layer includes a wide region in which the defect density is further lower.
(4) In the descriptions of the first to sixth aspects of the present invention, xe2x80x9copening areaxe2x80x9d means the area of the opening at the upper end of each pit, and xe2x80x9cdepthxe2x80x9d means the greatest depth of each pit measured from the upper end of each pit in the vertical direction.
(5) Preferably, the first to sixth aspects of the present invention may have one or any possible combination of the following additional features (i) to (ix).
(i) The plurality of pits are distributed over the upper surface of the first GaN layer with a density of 2.5xc3x97105 to 2.5xc3x97109 cmxe2x88x922.
(ii) The opening area of each of the plurality of pits is equal to or greater than 2.5 xcexcm2.
(iii) The distance between the closest ones of the pits is 0.1 to 10.0 xcexcm.
(iv) Horizontal cross sections of each of the plurality of pits may have an arbitrary shape. For example, the horizontal cross sections of each of the plurality of pits may have a round or polygonal (e.g., square or equilateral hexagonal) shape. The shapes and opening areas of the plurality of pits may be either identical or different.
(v) Vertical cross sections of each of the plurality of pits need not necessarily have a rectangular shape. For example, the bottom of each pit may be inclined (e.g., an inclined facet may constitute the bottom).
(vi) The plurality of pits may be regularly arranged, for example, with threefold, sixfold, or fourfold symmetry. Alternatively, the plurality of pits may be arranged randomly.
(vii) The processes according to the first and second aspects of the present invention may further comprise a step for forming as an uppermost layer a conductive GaN layer which is doped with a conductive impurity.
(viii) In the processes according to the first and second aspects of the present invention, it is possible to form a GaN film on a base substrate made of a material chosen from sapphire, SiC, ZnO, LiGaO2, LiAlO2, ZrB2, GaAs, GaP, Ge, Si, and the like, and form the first GaN layer on the GaN film. In this case, the base substrate may be removed after the formation of the uppermost GaN layer. Further, it is possible to remove layers from the base substrate up to any layer other than the uppermost GaN layer. For example, it is possible to remove all the layers from the base substrate up to the layer immediately below the uppermost GaN layer, and use only the conductive GaN layer as a substrate.
(ix) In each of the semiconductor elements according to the third and fourth aspects of the present invention, it is possible to remove layers of the substrate from the bottom (e.g., from a base substrate when the substrate is formed on the base substrate) up to any layer other than the uppermost GaN layer.